Geotechnical Report Dec 2013 Revision 1

                              55 LAKE MACDONALD DRIVE, COOROY


        ENGINEERING ASSESSMENT
6.1      Earthworks
Advice provided to Tectonic by the client indicates that earthworks will comprise the placement of up to
approximately 0.6m of fill across the lower lying areas of the site to raise the ground levels about 0.5m above
the Q100 inundation level.

Filling in excess of 0.6m is expected in the area of the proposed new road into Stage 3, where the ground
level drops about 0.5m towards the drainage gully (e.g. location of test pit TP8).

Excavations to depths of up to about 2.5m are expected for the sewer line on Stage 4.

Excavations in the drainage gully areas as stripping and clearing is proposed, together with placement of rock
spalls, topsoil and muclh.

Additional earthworks may occur at future residential areas to prepare level ground for slab-on-ground
construction.

6.1.1        Site Preparation and Traffickability
The general soil profile comprised about 0.2m of grey silty sand topsoil over about 0.2m of loose to medium
dense grey silty sand (similar material to the topsoil, but lacking roots). Underlying the silty sand is loose to
medium dense clayey sand, then high plasticity sandy/silty clay. At test pit TP1, TP8 and TP11, the clayey
sand layer was not encountered.

Stripping depths to remove the surface vegetation layer are expected to be typically about 0.2m deep. Deeper
stripping is expected in the area of the draining gullies. Two test pits excavated in or close to the drainage
gullies (TP8 and TP11) indicated yup to about 0.4m to 0.5m of stripping may be required in those areas.

The subgrade materials exposed after site stripping are expected to comprise predominantly grey, loose to
medium dense silty sand. Whilst in a dry condition (as experienced at the time of the investigation), the silty
sand forms a suitable subgrade for pavement construction and/or further filling. However, this material will be
subject to significant strength loss from repetitive vehicle loading (particularly from rubber tyred plant) or if they
become wet. If earthworks are undertaken during or following wet periods the grey silty sand may need to be
overexcavated and replaced with a suitable fill material.

If the silty sands are to be overexcavated, the exposed material is expected to comprise loose to medium
dense clayey sand. Laboratory tests indicated CBR values of 17% and 19% for that material, with field
moisture contents either just below the optimum moisture content (OMC) at test pit TP7, or at OMC at test pit
TP9. Some moisture conditioning (addition of water or drying) is to be expected prior to placement of
pavement materials.

In future residential areas, the potential subgrade materials are expected to be of suitable strength to allow for
adequate compaction of future fill to be carried out (provided silty sands are not wet).

At test pit TP8, the soil profile comprised very loose, high organic silty sand over wet, very loose sand, then
high plasticity silty clay. The very loose silty sand and sand are not considered suitable for subgrade platforms
and will be required to be excavated. Following removal of the silty sand and sand, it is expected that the
exposed silty clay will be wet, and will require conditioning (tyned, dried and recompacted) prior to placement
of fill.

At test pit TP12, and to a lesser extent at TP13, large logs were found in the topsoil layer, to a depth of about
0.4m BGL. This material must be fully removed prior to placement of fill.

Due to the potential for poor traffickability across the site during and after wet periods, placement of a
minimum 150 mm thick granular working platform may be required, particularly along haul roads and any
laydown areas.

After stripping of topsoil, the natural subgrade should be ‘proof rolled’ using a minimum 10 tonne drum roller in
order to identify any ‘soft spots’. Treatment of any ‘soft spots’ may involve excavation and replacement with



2 December 2013
Report N0. 13008-001-Rev1                                10